This invention relates to a slit scanning document illumination system which includes a facetted reflector opposite the illumination source.
In slit scanning photocopying systems, a common requirement is for relatively balanced illumination from both sides of the exposure slit. The exposure slit width is a system constraint which essentially determines the required shape of the illumination profile. Thus, the profile for a narrow exposure slit should be strongly peaked at the slit position while a wide exposure slit is compatible with a wide illumination source aperture.
It is known in the art how to enhance the required reflector opposite a scanning lamp. See, for example, U.S. Pat. No. 3,982,116 which discloses the use of an elliptical-type reflector opposite a scanning lamp.
In co-pending application, Ser. No. 802,750 assigned to the same assignee as the instant application, there is disclosed the use of a multi-segmented, or "Fresnel" reflector opposite a scanning lamp. This reflector optically simulates a cylindrical reflector by orienting a large number of plane mirror segments (in the order of 100 segments per inch) parallel to tangents of the cylindrical surface to be simulated. When subjected to the same physical constraints as its cylindrical counterpart, the Fresnel reflector exhibits improved illumination efficiency at the scan strip area.
These prior art reflectors have certain disadvantages. The cylindrical-type reflectors are subject to aberrations which cause light loss at the object plane. The Fresnel reflector, to the degree that simulation is successful, also suffers from the same aberration as the conic section it is simulating. The design of these prior art reflectors is also complicated by the space constraints imposed by the particular system geometry. Generally, an iterative, time-consuming optimization procedure is required during the design phase.
The facetted reflector of the present invention is distinguishable over the prior art by having a relatively small number of facets, each facet having a specific tilt angle and length such that at a given point on a given scan line the source is visible, by reflection, from every point on every facet.
The design of the facetted reflector is optimized by use of reflector design equations which incorporate the geometrical constraints of the particular illumination system. Since the facets are not designed to simulate any conic section, they are not subject to aberrations and the attendant loss of light. Additionally, the present reflector eliminates the facet "shadowing" at the design point which exists in the Fresnel reflector thereby rendering even greater illumination efficiency when comparing reflectors having the same system constraints.
It is an object of the present invention to provide a scanning illumination system which provides maximum possible illumination at a given object illumination scan area within given system constraints.
Another object is to facilitate and optimize the design of a facetted reflector.
Another object is to provide a reflector for a scanning illumination system which is not subject to aberrations at the design line.
A further object is to provide a scanning illumination system which will provide more intense illumination over a narrow exposure slit.
A still further object is to provide a facetted reflector requiring relatively few specularly reflecting facets.